I have always been of the belief that one should put their money into their glass, especially now that the cameras are pretty much all good. I am also guilty of spending the money for the pro long lenses. Here is my rationale:

IQ is extremely important. Fast glass is very important, focusing speed, and resolving power is critical. Without these qualities, you are compromising your ability to eliminate marginal. The difference between a marginal shot and a great shot is make or break. Yes, you can bump up your iso to get the speed you want. There is software that can clean up a dirty image, but there is a reason that camera and lens companies build these lenses and people buy and carry them. They are huge, heavy and expensive.

I used to love my Nikkor 180-400 f/4. Paid $13,000 for it. Magnificent lens, until I upgraded my camera
To the Z9. I found that the resolving power of the 180-400 did not match the capabilities of the camera. I noticed the difference and became unsatisfied. Still a great lens and the guy I sold it to got a great deal on a wonderful lens.

One important thing I have noticed, is that each of these lenses comes with a learning curve. Don’t expect that just because you invest tons of money on piece of equipment, it will instantaneously solve the problems and make you a great photographer. While it certainly does greatly enhance the output and ability to capture those great images, knowing how to use them is a critical part of the equation. I have a beautiful set of golf clubs, and I still can’t break 100! (On 18 holes, I’m not that pathetic 😂)

Of course, having said all of this, it ultimately comes down to three things, what you are going to use it for, your commitment to photography and how deep your pockets are.

On another note, I used to shoot JPEG. Captured a bunch of images that were quite good, then I discovered the differences between JPEG and RAW. (Not trying to lead this conversation into that age old debate) Most of the time, if your exposure is ok, JPEGs are fine, but when they are not, that’s when you really wish you had all of the extra information that RAW provides. My point is, why limit your potential when you can arm yourself with everything you can to capture exceptional images?

Sony & Nikon & Canon put out 600mm prime lenses in the $13,000 range. I can get a Fujifilm or Tamron or Sigma 150-600 mm lens for $2000 or less. I would love to see the same distant object taken at 600mm with one of the primes and one of the lesser telephotos at differing levels of cropping to see the difference in detail/sharpness. Any website show this or does someone here have such photos? I am not expecting a dramatic difference as I understand you pay a high premium for incremental improvements generally. I am not planning on any $13K purchase (unless I win Mega Millions lottery), but just curious.

Sony & Nikon & Canon put out 600mm prime lenses in the $13,000 range. I can get a Fujifilm or Tamron or Sigma 150-600 mm lens for $2000 or less. I would love to see the same distant object taken at 600mm with one of the primes and one of the lesser telephotos at differing levels of cropping to see the difference in detail/sharpness. Any website show this or does someone here have such photos? I am not expecting a dramatic difference as I understand you pay a high premium for incremental improvements generally. I am not planning on any $13K purchase (unless I win Mega Millions lottery), but just curious.

Sony & Nikon & Canon put out 600mm prime lenses in the $13,000 range. I can get a Fujifilm or Tamron or Sigma 150-600 mm lens for $2000 or less. I would love to see the same distant object taken at 600mm with one of the primes and one of the lesser telephotos at differing levels of cropping to see the difference in detail/sharpness. Any website show this or does someone here have such photos? I am not expecting a dramatic difference as I understand you pay a high premium for incremental improvements generally. I am not planning on any $13K purchase (unless I win Mega Millions lottery), but just curious.

Fast primes are the standards of comparison !......I CAN tell you that a Canon 300 2.8 and 2X is BETTER than my Tamron 150-600 G2 both lenses wide open @600 !

Starting back in the film days, light intensity at the film was very important because of the relative in-sensitivity of film. Light intensity ( lens aperture speed) and shallow DOF has always been important for focusing. Today as digital sensors and processing become better and AF improves, the advantages of the fast primes are becoming more diminished. But there will always be a pure native optical IQ advantage of current primes.

YOU will not see that much difference. For example, I shoot with both the Sony 200-600 and 600 f4. If I take the same bird with both lenses wide open the back ground on the 600 f4 will be more out of focus than with the 200-600. Now the 600 will focus a nano second faster but in most cases YOU will not notice the difference for the price.
The 600 will acquire focus nano seconds faster than the 200-600, YOU will not notice a difference for the price.
In early morning light the 600 will lock on nano seconds before the 200-600, YOU will not notice the difference for the price.
If both images are put into post processing and enlarged 10 times, the 600 will hold the image better than the 200-600, but with Topaz Denoise and Sharpen YOU may not notice the difference for the price.
If you have the money for the 600, I say go ahead and buy it, as a former professional photographer, I like the results I get with the 600 over the 200-600.
The 600 does not focus as close as the 200-600, if a bird is close I might be out of luck with the 600. So I usually carry the Sony 70-200 for closer birds. My 70-200 is strapped to my waist with a Spider holster.
Now the new Nikon 180-600 will focus very close, I have ordered it through Nikon Professional Services, I have not tested it but I will compare results against my Sony 600.
For the price I like Sony, Nikon, and Canon glass for my tele zooms, I generally stay away from third party glass from Tamron and Sigma, for the price point there is a difference in price that favors the quality and manufacturing process of Sony, Nikon and Canon over third party glass.
So, bottom line, if I were you I would buy the Fuji 100-400, the price point is excellent and it is lighter than most 600 primes. I have seen the results with the Fuji 100-400.
https://www.bhphotovideo.com/c/product/1210897-REG/fujifilm_16501109_xf_100_400mm_f_4_5_5_6_r.html
But I will stick with my full frame camera's sensor over the 4/3 sensor. Personal choice here.